Steel holder block for plastic molding

ABSTRACT

A manufacturing mold base for plastic injection molds is formed from a martensitic stainless steel alloy comprising: about 0.03%-0.06% by weight C, about 1.0%-1.6% by weight Mn, about 0.01%-0.03% by weight P, about 0.06%-0.3% by weight S, about 0.25%-1.0% by weight Si, about 12.0%-14.0% by weight Cr, about 0.5%-1.3% by weight Cu, about 0.01%-0.1% by weight V, about 0.02%-0.08% by weight N, with the balance being Fe with trace amounts of ordinarily present elements.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] This invention relates to a specific class of martensiticstainless steel designed and optimized for use in building mold basesfor plastic injection mold tooling.

[0003] 2. Description of Prior Art

[0004] Mold bases for plastic injection molds have evolved withincreasing sophistication as the uses of and demands on plastics havegrown. Initially these tools were constructed mainly of carbon and lowalloy steels. New plastics, higher operating stresses, larger highcavitation tooling, longer production runs and inherent corrosionproblems with the application led to the use of stainless steels inincreasing quantities since around 1980, however the stainless steels inpredominant use have many undesirable features.

[0005] The stainless steels in use for this application are essentiallymodifications of a standard AISI 420 stainless steel (S42000), in somecases with sulfur additions to enhance machinability. No suitablestainless grade for this specific application was ever developed. Majordisadvantages of the current grades are:

[0006] (a) Very high hardenability, capable of developing Rockwell-Chardness in excess of 50 Rockwell-C. The application typically onlyrequires hardness in the range of 32 to 39 Rockwell-C. The extremehardenability demands extensive thermal processing at the ingotmanufacture stage, hot working stage and heat treating stage to preventcatastrophic cracking in the raw product. Lengthy annealing cycles arerequired to render the material safe to handle and soft enough forflattening and saw cutting. Rapid change in hardness at the requiredtempering temperatures, resulting in non-uniformity of hardness. Inbrief, these steels have too much hardenability and too little ductilityfor the application.

[0007] (b) Poor formability characteristics, making it difficult toachieve the degree of flatness desired in the product. Inherentstiffness and lack of ductility make the product unsuitable for hot orcold flattening and often leads to product breakage while attempting toflatten.

[0008] (c) High residual stresses developed during hardening leads todimensional instability during complex machining operations, andespecially if large cavities are formed to accommodate large molds,

[0009] (d) Poor surface quality as hot worked. This feature requiresmaking product at oversize gauges to allow for proper surface cleanup inthe final application.

[0010] (e) Poor weldability, making repairs difficult and unreliable.

[0011] (f) Only moderately resistant to corrosion.

[0012] (g) “Breakout” at the exit surfaces when drilled. Breakout occurswhen the metal surrounding the drill exit area fractures and tears awayin advance of the drill, creating a ragged hole edge.

[0013] (h) Extensive edge tearing during hot working, resulting in pooryields and also necessitates expensive edge trimming prior to hardening.

OBJECTS AND ADVANTAGES OF THE INVENTION

[0014] The material of the invention has been developed specifically forthe plastics mold base industry. Every effort was made to optimize thosequalities which are known to be important for both manufacturing theproduct and for successful operation of the equipment in which theproduct is used. Laboratory scale, pilot scale and full scale productionmelts were produced and evaluated in developing this invention. Theimportant features of the alloy and the elements of the invention whichproduce these features are as follows:

[0015] (a) The chemical analysis of the invention simplified thermalprocessing of the cast ingot, resulting in cost and time savings. Ingotsrequire only slow cooling after stripping, not a lengthy and costlyfurnace annealing treatment.

[0016] (b) The chemical analysis of the invention is designed to yieldacceptable hardness for the intended application either as hot worked,as hot worked and stress relieved, as normalized or as normalized andstress relieved. No post hot working annealing cycles are required toprotect the material from cracking.

[0017] (c) The inherent ductility of the invention virtually eliminatesoccurrences of drill breakout.

[0018] (d) The chemical analysis of the invention, specifically the lowcarbon level, and high chromium level and addition of copper result inimproved corrosion resistance.

[0019] (e) The inherent ductility of the invention eliminates alloccurrences of drill breakout.

[0020] (f) Excellent hot worked surface quality of the invention permitsa reduction in over gauge allowances necessary to meet final productsizes. This reduces machining and grinding costs and increases overallyield from ingot to plate product.

[0021] (g) Reduced hot strength of the invention results in moreeffective flattening during hot working, yielding excellent as rolledflatness totally free of waviness and wrinkles. This eliminates the needfor secondary cold or hot flattening operations and the improvedflatness reduces the amounts of machining required to produce clean,bright metal finished surfaces.

[0022] (h) High ductility of the invention provides a material which canwithstand more than twice the amount of cold deformation prior tofracture compared to current modified AISI 420 grades. High ductilityalso allows increased levels of Sulphur without danger of hot tearing.The increased Sulphur content produces excellent machinability.

[0023] (i) Calcium addition to the molten metal results in controlledsulfide morphology, eliminating objectionable stringer type sulfides.

[0024] (j) Thermal conductivity of the invention has been improved byreduction of chromium and addition of copper.

[0025] (k) Low carbon level and reduced hardenability result in areadily weldable material.

SUMMARY OF THE INVENTION

[0026] The invention provides an improved stainless steel for theplastic injection molding industry designed specifically to fit theapplication's requirements for strength, weldability, machinability,flatness, corrosion resistance, conductivity and surface quality. Theinvention removes the need for lengthy thermal processing duringmanufacture and eliminates the occurrences of failures and productlosses due to low ductility and crack sensitivity of other gradescommonly used for the application.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic perspective view of a rectangular platemanufactured from the present invention;

[0028]FIG. 2A is a schematic planar view of a mold base manufacturedfrom the rectangular plate shown in FIG. 1; and

[0029]FIG. 2B is a schematic cross-sectional view of the mold base shownin FIG. 2A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] The invention is a martensitic stainless steel with less than 10%ferrite phase and chemical composition as shown in Table I. The materialis electric furnace melted and further processed by AOD, VOD or othersuitable means for producing low carbon stainless steels. The materialis calcium treated to provide optimum control over the manganese sulfidemorphology. The composition range of this invention is given below inTable I. TABLE I Composition of Improved Steel Element C Mn P S Si Cr CuVa N Minimum .03 1.00 .010 .06 .25 12.00 .50 .01 .02 Maximum .06 1.60.030 .30 1.00 14.00 1.30 .10 .08 Typical .04 1.20 .020 .20 .40 12.50 .80.04 .05

[0031] The balance of the invention's composition is 80% or more Fe andthose impurities and tramp elements which are inevitably included duringthe melting of the material. The function of each of the intentionallyincluded elements in the analysis are as follows:

[0032] Carbon—0.06% Maximum

[0033] Carbon combines with chromium to precipitate as a carbide,depleting the effective level of chromium which negatively affectscorrosion resistance. Carbon level dramatically controls hardnessattainable. Maintaining the carbon level of the grade as low as possiblewhile still achieving the designed hardness levels promotes improvedcorrosion resistance with addition of a minimum of chromium. Carboncontent of 0.06% or less provides adequate hardenability withoutdegrading the corrosion properties of the grade and so is thusspecified.

[0034] Manganese: 1.00 to 1.60%

[0035] Manganese acts as a strengthening agent, a de-oxidizer and also,as an austenite stabilizer, prevents the formation of ferrite phase. Theprimary importance, however, of manganese in this grade is the formationof sulfides which prevent hot working problems, normally associated withhigh sulfur content without adequate manganese present in the analysis.The upper limit of 1.60% manganese is specified to control theembrittling effects of excess Manganese. The specified range of 1.00 to1.60% manganese produces all the desired effects without any negativeimpact on the grade's properties.

[0036] Phosphorus: 0.010 to 0.030%

[0037] Phosphorous adds to the hardenability of steels, but is normallyreduced to the lowest levels possible due to causing brittleness. Forthis application, the phosphorus is intentionally not reduced toextremely low levels. The range of about 0.01 to 0.03 phosphorous wasspecified to take advantage of phosphorus' slight contribution tocorrosion resistance but more importantly for its positive affects onmachinability.

[0038] Sulfur: 0.06 to 0.30%

[0039] Sulfur is the most widely used additive to steel to promoteimproved machinability and is specified in this steel for that reason.Sulfur at the specified level has been found to be effective in makingthis alloy readily machinable by all standard processes, but remains inbalance with the rest of the analysis to the level that hot workingproperties, toughness, ductility and corrosion resistance remainacceptable.

[0040] Chromium: 12.00 to 14.00%

[0041] Chromium acts to enhance hardenability, making possible amaterial which will readily transform to the desired martensiticstructure in heavy cross sections with air cooling. Chromium content of12% minimum is provided to give sufficient corrosion resistance in thegrade. Increasing levels of chromium promote the formation of theundesirable ferrite phase, particularly in this grade with low carboncontent. The chromium is therefore controlled to the range of 12%minimum to 14% maximum.

[0042] Silicon: 0.25 to 1.00%

[0043] Silicon acts as the primary de-oxidizer in the molten metal andis therefore necessary. Increasing levels of silicon however produceferrite. Adequate de-oxidizing action occurs with silicon present in therange of 0.25% minimum to 1.00% maximum and silicon is therefore limitedto this content in the alloy.

[0044] Copper: 0.50 to 1.30%

[0045] The addition of copper is a unique feature of this alloy for itsintended application. At this level, copper is fully dissolved in thebase metal matrix as a solid solution. The presence of copper improvesthe corrosion resistance and conductivity. Additionally, the copperallows the material to respond to a low temperature aging process whichcan be used to elevate the strength level of the material either priorto machining or after, with no apparent distortion or cracking problems.Lower levels of Copper than specified diminish the desired effect andhigher levels of Copper can promote hot working problems. The range of0.50 to 1.30 has been found to produce the planned results with nodetrimental effects and so is specified as shown.

[0046] Nitrogen: 0.02 to 0.08%

[0047] Nitrogen contributes to the corrosion resistance of the materialand also acts to stabilize the austenite phase, improving hardenabiltyand diminishing the occurrence of a ferrite phase. Nitrogen tends toform chromium rich nitride particles during aging and tempering. Theseparticles reduce the effectiveness of the chromium from the standpointof corrosion resistance. Therefore, the amount of nitrogen added is keptmoderate within the 0.02 to 0.08% range specified.

[0048] Vanadium: 0.01 to 0.10%

[0049] Vanadium forms a stable carbide precipitate which is veryeffective in controlling grain growth, necessary to produce materialwithout grain coarsening which would promote unacceptable low ductility.Due to its tendency to increase the formation of the ferrite phase andin light of the low carbon levels in the material, vanadium level isadequate and useful at the specified range of 0.01 to 0.10%

[0050] Calcium injection in the molten metal provides shape control ofthe manganese sulfide inclusions in the hot worked steel. Shape controlis essential to provide uniform distribution and effectiveness of theseinclusions from the machinability aspect of its properties. Elongatedsulfides, typical of noncalcium treated, resulfurized steels, can leadto poor surface quality in machining, drill breakout and in general poorductility, especially in directions transverse to the primary hotworking direction.

[0051] Details of Manufacturing

[0052] Material is produced as a low carbon stainless steel by electricfurnace melting, post melt refining, deoxidation, alloy trimming, sulfuraddition and calcium injection treatment. Molten metal is cast intometal ingots by bottom pouring methods. Following solidification, ingotsare stripped from the molds, slow cooled and then 100% surfaceconditioned by grinding to prepare the metal for subsequent hot workingoperations.

[0053] Hot working is provided by either rolling or forging or acombination of both. In all cases, transverse hot working is utilized tominimize any directionality of mechanical properties in the material.Hot working is performed within the temperature range of 1700-2150° F.All material is flattened immediately at the conclusion of hot workingwhile the metal is still hot. Product width to thickness ratio iscontrolled to assure that the producing facility has adequate power toeffectively flatten the product. The invention allows free air coolingof the material after hot working without any precautions or thermalprotection procedures. This practice results in time and cost savingsand promotes improved flatness as the product can remain at rest oncooling beds until rigid.

[0054] Material hardness is designed to be controlled by the analysis ofthe melt, as opposed to reaching the desired hardness level by“tempering” back from an over hardened state as is done with traditionalgrades for this application. The invention provides a grade that willmaintain a stable hardness profile as rolled or as normalized. Theaddition of a low temperature stress relieving treatment in the range of450° F. to 650° F. acts to improve material ductility and increasedimensional stability without changing the as rolled or as normalizedhardness. A hardness increase of 4 to 6 Rockwell-C can be achieved byaging the material between 700° F. and 900° F. Material can be renderedvery soft, i.e., below Rockwell-C 20 hardness by over tempering in the1200° F. to 1300° F. range if necessary to facilitate forming orflattening. Reversion to the planned hardness level is easilyaccomplished by normalizing the material at temperatures of 1650° F. orhigher. Plates of the improved alloy may be provided with a standardheat treatment to a hardness of 32-36 HRC (BRN 301-340). Hardness abovethe standard range is readily obtainable with simple low temperatureheat treatment. The maximum hardness attainable is about 40 HRC.

[0055] Due to the controlled hardenability, welds and weld repairs canbe made on the material of this invention with no concerns of welddefects nor of deleteriously altering the properties of the basematerial.

[0056] Referring to FIGS. 1-2, the invention is designed for use inmanufacturing mold bases for plastic injection mold tooling. FIGS. 1 to2B are presented to illustrate the key features of such a product.

[0057] As is illustrated in FIG. 1, the majority of the product forwhich the invention is designed for begins as a piece of saw cutrectangular plate 100, which is milled and/or ground to provide sixsmooth surfaces at specified dimensions as in the manner shown inFIG. 1. The excellent flatness of the invention as produced minimizesthe amounts of material that must be removed from surfaces 101 and 102to produce flat, parallel clean metal surfaces.

[0058]FIG. 2A-2B show an exemplary mold base 104 manufactured from therectangular plate 100. The mold base 104 is configured to have arectangular cavity 105 (i.e., main pocket), typically having a flatbottom surface 106. During the manufacture and operation of the moldbase 104, it is essential that surfaces 101, 102 and 106 remain parallelat all times. This is difficult when the cavity 105 is large as theremoval of the material can warp the plate 100 if the material is notdimensionally stable. For example, parallel surfaces 104, 106, withinabout 0.005 to 0.010 inches across 24 inches is desired, and theimproved alloy is believed to achieve parallel surfaces within a fewthousandths of an inch across 24 inches. The invention thus provides astable material that does not significantly distort during machining,even after heavy metal removal such as is performed when a “Main Pocket”105 is formed.

[0059] Referring to FIGS. 2A-2B, the mold base 104 which the inventionsupports also requires many machined holes such as guide pin holes 110,bushing holes 111, ejector pin holes 112, cooling channels 113 andothers. The invention permits rapid machining of holes with no danger ofbreakage around the hole exist area, due to the invention's inherentductility and excellent quality of internal threads.

[0060] Regardless of technology, equipment and controls exercised inmachining, errors inevitably occur which require repairs or replacementof machined components. The excellent weldability of the inventionallows for weld repairs on the mold base 104.

[0061] Hot Work Details

[0062] The following table presents data from rolling standard 22″×56″plate ingots of “20FM” (typical of 420 stainless base “prior art”) andthe alloy of the present invention, approximately 16,000 lbs. each onthe 140″ plate mill, Lukens Steel, Coatesville, Pa. All plates wererolled to 98″ wide: ″20FM Rolling Data Inventive Alloy Rolling DataAverage Average Average Average Final Plate Total Pressure ReductionPlate Total Pressure Reduction Gauge No. Passes k-lbs per Pass No.Passes k-lbs per Pass 1.165 9F0402-3 41 4604 .522 9Y3274-2 35 2936 .5822.135 9F0402-5 41 4482 .485 9Y3274-3 31 2819 .641 2.625 9Y1183-3 41 4668.473 9Y3274-5 27 3076 .718

[0063] This data indicates that as expected, the applicants new alloyrequires fewer passes and lower rolling forces than 420 type materials.

[0064] Hot work conditions for 420 type stainless and the material ofthis invention are the same. In general, material is heated to 2150° F.,held sufficiently long to “soak” through the cross section, and thenrolled or forged. Rolling or forging is suspended when materialtemperature drops to 1700° F. The major difference is in time to heatingots. 420 type ingots have a heating cycle of 48 hours and requirecharging into a relatively low temperature furnace or pit in order toavoid thermal cracking during heating and heating rates are lower forthe same reason. Ingots of the improved steel material, 22″×56″, wereheated in 24 hours.

[0065] Plates of the improved material have shown excellent flatness asrolled and hot leveled (this hot leveling is an in-line operation at therolling mill, done within minutes of final reduction pass on the mill).Because the improved plates can go cold with no danger of cracking, theyare left to cool until rigid before lifting (prevents sagging andbending) and we are seeing flatness of better than ¼″ across 12 footspans. The improved material shows little resistance to leveling at thehot mill leveler and waves and ripples can be removed effectively. 420type material on the other hand has higher hot strength and is veryresistant to effective leveling at the hot mill, which results in mostplates having some unacceptable areas of sweep, waves and/or ripples. Inaddition, 420 type must be picked up off the cooling tables, put intopiles, covered and slow cooled to prevent cracking. This moving whilehot and the irregular support offered by the random stacking has alwayslead to bending of plates, which then must later be flattened. Annealingis also done in irregular piles which tends to yield sagging ends whenpiles are not built with longest plate on the bottom, next longestsecond, etc. The secondary flattening, done by roller leveling up to 2⅝″gauge and gag press above 2⅝″, is not very effective as it can only bedone with the plates warm (300-400° F.) and plates often either springback to an out-of-flat condition during subsequent heat treating or evenbreak catastrophically during the flattening operations.

[0066] Thermal conductivity of the alloy is adequate for its intendedapplications and is comparable to 420 type stainless materials.

[0067] Corrosion has been evaluated first by placing polished samples of420 type material and the Applicants' improved material near theseashore and visually evaluating after several weeks exposure. Theimproved material of this invention showed little, if any, effect of theexposure, while the 420 type developed rusting. The second evaluationwas to machine full-size mold plates from both types of material andallow them to sit outdoors (mold system storage without rusting is abenefit to users) unprotected. Plates made of the improved material ofthis invention have been exposed over six months now and show norusting, while the 420 material has become generally rusty and corroded.Again, these are qualitative rather than quantitative, but in the moldbusiness, sophisticated testing is not performed or required, as moldsare typically exposed to cooling water or the weather and humidity, andseldom exposed to any serious chemical systems, such as chlorides.

[0068] Exemplary mechanical and physical properties are given below:Typical Mechanical Properties - Improved Alloy vs. 420 Type MaterialGrade UTS-ksi .2% YS-ksi % EL in 2″ % RA 420 Type SS 155.0 132.0 10.020.0 Applicants' 155.0 127.0 12-14 about 35.0 Improved Alloy

[0069] PHYSICAL DATA Prehardened to 321 HB. Data at room and elevatedtemperatures. Temperature 68° F. 390° F. (20° C.) (200° C.) Density,kg/m³ 7,800 7,750 lbs/in³ .282 .280 Modules of elasticity N/mm² 20,00019,000 psi 29.0 × 10⁶ 27.6 × 10⁶ Coefficient of thermal expansion per °F. from 68° F. —  6.1 × 10⁻⁶ per ° C. from 20° C. — 11.0 × 10⁻⁶

[0070] Actual machining trials have shown that the improved material ofthis invention machines easily (with equal or less horsepower) than 420types at the same hardness levels, and yet gives better surfacefinishes, excellent quality of drilled and tapped hole threads and not asingle incidence of material breakout (fracture) at the exit side of aplate when drilling large diameter holes at high rates of speed.Breakout is common in all 420 types and was one of the key reasons fordeveloping this improved material. The behavior of this improvedmaterial is a direct result of higher sulfur (for improved chipbreakage) yet with better material ductility.

[0071] Better weldability of the improved material of this invention vs.a 420 type stainless is a given. Enough history exists to show that asteel with 0.04% C and 13.0% Cr will be vastly superior to a 0.32% C and16.0% Cr steel. The extreme hardenability of the latter leads tocracking and high hardness (55 Rockwell-C in the heat affected zone vs.35 Rockwell-C in the original base material), while the improvedmaterial is designed to produce its usable hardness of 34-36 Rockwell-Ceven as normalized, and therefore the heat affected zone does notoverharden. A slight hardness increase is experienced in the improvedmaterial due to the aging effect, but this is typically a 2-4 Rockwell-Cincrease and is accompanied by increased ductility which yieldsexcellent welds with no cracking. The hardness is believed uniform andconsistent in all directions.

[0072] The improved alloy may be easily welded with no preheat or postheat treatment. The heat affected zone does not exhibit extremely highhardness as with the prior art 420F and other high carbon metals.Further, the improved alloy has significantly reduced the risk ofcracking during and after welding, to an almost negligible level.Moreover, photomicrographs reveal a uniform hardness throughout theweld, heat affected zone and base metal. The above advantages assume thewelding electrodes are of the same alloy as the improved material ofthis invention.

[0073] The resulting alloy of this invention has increased toughness anduniformity which reduces the risk of breakage caused by machiningpressures and stresses. The material also reduces the risk of platecracking during the entire life of the mold. Charpy V-notch (CVN) impacttests at room temperature of the improved material have an average CVNof about 16, compared to a CVN of 8 for 420F steel. A 12% elongation in2″ is significantly more than the 9% comparable elongation for 420Fsteel. Similarly, a 32% RA for the improved alloy is a considerableimprovement over the 20% RA for the 420F alloy. Typical impactproperties of the improved alloy are given below. IMPACT STRENGTHLongitudinal Charpy V-notch Tests from a 3″ (76 mm) rolled plate at 321HB Testing temperature 68° F. (20° C.) 390° F. (200° C.) Ft-lbs 16 26Joules 22 36

[0074] The increased hot ductility of this alloy promotes smoother “asrolled” plate surfaces that are free of tears and cracks, thus reducingthe possibility of cracks remaining after machining. The improvedsurfaces mean less stock removal to produce finished sizes, resulting inless machining and lower material costs.

[0075] the foregoing fully reveals the gist of the present invention sothat others can, by applying current knowledge, readily adapt it forvarious uses without omitting features that, from the stand point ofprior art, fairly constitutes essential characteristics of the genericor specific aspects of this invention, and, therefore, such adaptationsshould and are intended to be comprehended within the meaning ad rangeof equivalence of the following claims.

What is claimed is:
 1. A manufacturing mold base for plastic injectionmolds having sufficient dimensional stability that at least two opposingsurfaces remain substantially parallel during formation of the base andformation and any subsequent heat treating of a major cavity in thebase, the base having suitable ductility and breakout resistance forrepeatable formation of one or more apertures in the mold withoutbreakout, which apertures render the base suitable for use as a moldbase, and further having suitable corrosion resistance and weldabilityfor use as a mold base, the base being formed from a martensiticstainless steel alloy comprising: about 0.03%-0.06% by weight C; about1.0%-1.6% by weight Mn; about 0.01%-0.03% by weight P; about 0.06%-0.3%by weight S; about 0.25%-1.0% by weight Si; about 12.0%-14.0% by weightCr; about 0.5%-1.3% by weight Cu; about 0.01%-0.1% by weight V; about0.02%-0.08% by weight N; and the balance being Fe with trace amounts ofordinarily present elements.
 2. The mold base of claim 1, wherein saidalloy has a 2% yield strength of at least 115 ksi and an ultimatetensile strength of at least 145 ksi.
 3. The mold base of claim 1,wherein said alloy has an elongation in two inches of 14% and areduction in area of 35%.
 4. The mold base of claim 1, wherein saidalloy has a hardness within the range of 32 to 36 HRC.
 5. The mold baseof claim 1, wherein said alloy is normalized to provide more uniformalloy microstructure.
 6. The mold base of claim 1, wherein said alloycomprises a maximum of 10% by volume ferrite phase.
 7. A martensitictool steel alloy for use in the manufacture of plastic injection moldbase components, said alloy is comprised of about 1.0%-1.6% by weightMn, about 0.01%-0.03% by weight P, about 0.25%-1.0% by weight Si, about0.5%-1.3% by weight Cu, about 0.01%-0.1% by weight V, about 0.02%-0.08%by weight N, a maximum of 0.06% by weight C, a maximum of 14.0% byweight Cr, a maximum of 0.3% by weight S and the balance being Fe withresidual impurities wherein said alloy comprises a hardness within therange of 32 to 36 HRC.
 8. The alloy of claim 7, wherein said alloycomprises a 2% yield strength of at least 115 ksi and an ultimatetensile strength of at least 145 ksi.
 9. The alloy of claim 7, whereinsaid alloy comprises an elongation in two inches of 14% and a reductionin area of 35%.
 10. The alloy of claim 7, wherein said alloy isnormalized to provide more uniform alloy microstructure.
 11. A mold basemade of the alloy of claim
 7. 12. A process of manufacturing a mold basefor plastic injection molds, said process comprising the steps of: heattreating a steel alloy comprising about 1.0%-1.6% by weight Mn, about0.01%-0.03% by weight P, about 0.25%-1.0% by weight Si, about 0.5%-1.3%by weight Cu, about 0.01%-0.1% by weight V, about 0.02%-0.08% by weightN, about 0.03%-0.06% by weight C, about 12.0%-14.0% by weight Cr, about0.06%-0.3% by weight S and the balance being Fe with residualimpurities, at a temperature not lower than 2150° F.; hot working saidsteel alloy within the temperature range of about 1700-2150° F.; coolingsaid steel alloy by free air cooling to room temperature; and forming amold base from said alloy for use in plastic injection molding.
 13. Theprocess of claim 12, wherein said step of hot working said steel alloycomprises the steps of rolling or forging said steel alloy.
 14. Theprocess of claim 13, further comprising hot leveling said steel alloyafter finalizing said rolling or forging.
 15. The process of claim 12,further comprising the step of a stress relieving treatment performed ina temperature range of 450-650° F.
 16. The process of claim 12, furthercomprising the step of aging said steel alloy at a temperature range of700-900° F. to raise the HRC-hardness value of said alloy by 4 to 6hardness units.
 17. A mold base formed by the process of claim 12.